The invention relates to amplifiers, particularly to low noise, high slew rate CMOS operational amplifiers that includes class AB amplifiers.
High-speed operational amplifiers that are capable of amplifying both small and large magnitude signals with low distortion, low noise, good large-signal handling capability, and high slew rate generally are desirable in analog circuits. Various operational amplifier architectures having some or all of these properties are suitable for high-speed applications in analog circuits. The two main types of voltage feedback operational amplifiers are the folded cascode operational amplifier and the class AB operational amplifier. The folded cascode operational amplifier topology provides relatively low noise, low distortion, high gain, wide bandwidth and good DC performance, but also is characterized by a relatively poor slew rate. The class AB operational amplifier offers improved slew rate but limited large signal handling capability, poor noise performance, and poor DC performance.
The operational amplifier architecture shown in commonly assigned U.S. Pat. No. 6,710,654 entitled “Bipolar Class a Be Folded Cascode Operational Amplifier for High-Speed Applications” issued Mar. 23, 2004 to Parkhurst et al. (which is entirely incorporated herein by reference) greatly improves the linear range of the input signal, and also provides increased slew rate and reduced THD (total harmonic distortion). FIG. 1 shows a schematic diagram of an operational amplifier as disclosed in the foregoing '654 patent. That operational amplifier includes a first stage translinear transconductance circuit 2A, referred to in the '654 patent as a first stage comparator circuit. The translinear transconductance circuit 2A is utilized for biasing a second stage folded cascode amplifier circuit 3A to transform a hyperbolic tangent function to a hyperbolic sine function in order to provide an operational amplifier having low noise amplification with a high slew rate. The operational amplifier disclosed in the '654 patent uses a hyperbolic cosine (“cosh”) translinear transconductance stage to bias the input stage tail current transistors to transform the relationship of the tail currents with respect to the differential voltage input from a hyperbolic tangent (“tanh”) function to a hyperbolic sine (“sinh”) function. The resulting folded cascode operational amplifier, which is implemented using only bipolar transistors, achieves high slew rates without deleteriously affecting its low noise, low distortion, high gain, and wide bandwidth DC performance parameters.
There is an unmet need for an improved CMOS operational amplifier of the type including a class AB output stage which provides low noise and fast slewing.
There also is an unmet need for an improved CMOS operational amplifier of the type including a class AB output stage which provides low noise and fast slewing and also provides substantially improved large-signal handling capability, i.e., substantially improved output dynamic range.
There also is an unmet need for a CMOS operational amplifier having reduced quiescent current, low noise, and fast settling time for a large output voltage step.